Poco::DateTimeParser::tryParse issue

Hi all, if you use DateTimeParser::tryParse to parse an invalida datetime, often you obtain true but the datetime is invalid

```
    Poco::DateTime dt;
int tz = 0;
if (Poco::DateTimeParser::tryParse("%d/%m/%Y", "some invalid date time", dt, tz)) {
         //you obtain true but with dt with a negative value
}
```
